During the past two decades, the reverse osmosis (RO) process has gained extensive attention in the purification and preparation of drinking and ultrapure water for electronic and pharmaceutical industries and separation of organics from aqueous streams [see, for example, Murakami, S., et al., Polymer 43, 2117 (2002); Toki, S., et al., Macromolecules 35, 6578 (2002); Toki, S., et al., Polymer 44, 6003 (2003)]. However, successful utilization of RO technology has been hampered by membrane fouling, which creates a major obstacle for membrane applications from both technological and economic aspects of process utilization [see Toki, S., et al., Macromolecules 36, 5915 (2003); Trabelsi, S., et al., Macromolecules 36, 7624 (2003)]. Therefore, there is a growing interest in surface modification of existing RO membranes to introduce properties that markedly reduce fouling, specifically biofouling, while retaining permeate water flux and rejection characteristics of the commercially available membranes.
Commercial RO membranes are typically made of thin-film-composite aromatic polyamides for which the microorganism-induced biofilm formation on the surface is a major problem. This biofouling often results in up to 30-40% permeate flux declines, requiring for compensation increases in the operating pressures as high as 50%, regular cleaning by chlorine treatment and even complete replacements of the filtration module every 2-3 years. Table 1 below summarizes the salient features of representative commercially available RO membranes and shows that all of the polyamide and polyethersulfone ones are prone to biofouling.
TABLE 1Selected properties of some commercial RO membranesElix ® 3 ROMembraneHR98PPCA995PESEPA-MS05SEPA-SS1CDESAL-3BCartridgeManufacturing Dow/FilmTec Dow/FilmTecGE-OsmonicsGE-OsmonicsDesalinationMilliporecompanySystems, Inc.CompositionPolyamideDiacetate/PolyamideCellulosePolyether-Polyamidethin filmpolyesteracetatesulfonethin filmcompositecompositeTypical NaCl>97.5>95>98>9898.594-99rejection (%)ChlorineLowLowLowLowLowLowtolerancepH range2-112-8.53-112-84-11 4-11Tendency forHighLowHighLowHighHighbiofoulingMaximum87010151015101565065operatingpressure (psi)Maximum603580505035operating temp(° C.)
In fact, apart from the cost of energy to run the high-pressure pumps, membrane fouling is the single most important factor that controls the cost of RO water purification unit (ROWPU) operations. Since the maintenance and remediation expenses represent about 30% of the total operating cost, a new generation of membranes with inherent anti-fouling capability is urgently needed.
Clearly, it would be advantageous to provide an RO membrane that resists biofouling without compromising the separation properties of the membranes.